Ideas to include
The phrase she always says
Every mother has one. Text it to yourself before you forget it, then put it in the brief. It is the line that will get her.
From all the siblings
One song, everyone's name in it, one gift instead of three separate panics. Collect a memory from each and send them together.
For grandma too
Grandmothers are mothers, and often the ones who get a card at best. A song about the actual house, the actual kitchen, the actual summers.